> The second most dramatic changes in genetic expression occurs at the time the
> cell enters senescence. These have been discussed before and I won't bother
> with the details here.
This statement is fairly unsupported. Not nearly enough
work has been done to say whether this is true or not. Just
because it seems intuitive that between terminal
differentiation and visible senescence nothing much would
change (since we don't see phenotypic changes in culture),
doesn't mean that is what is happening. In fact, there are
some theories that would argue that aging is a continuous
change starting at the onset of maturity, and that the
visible manifestation of senescence is only seem once things
get REALLY out of control and cause the cell to function
improperly.
> Still, studies may show different genetic expression between young and old
> cells in the absence of the senescent state. (Although my brief Medline
> search couldn't come up with any, that is more a lack of knowing where to
> look rather than a lack of studies.)
Actually I think it is a lack of studies. The number of
genes that we have information on is so small (relative to
the number of total genes) that we really don't know what is
going on at this point.
Also bear in mind that the studies that do exist are studies
that are grabbing the low-hanging fruit. To think that the
majority of genes are going to have the same type of
expression pattern changes as the genes that we currently
know about may be completely wrong. It seems quite possible
to me that the only reason the genes that we know change
with aging were found was because they change dramatically
and so are easy to spot with techniques such as differential
display or subtractive hybridization. There could be
hundreds or thousands of smaller perturbations in gene
expression, going up, going down, or oscillating, starting
right after terminal differentiation, that are very
important, but we simply have not spotted them because the
technology to do so is only coming of age right now.